By Dan Hooper, Ph.D., University of Chicago
Albert Einstein was skeptical of the new interpretation of quantum theory that had emerged in the 1920s. In December 1926, Einstein wrote a letter to German physicist and mathematician Max Born, expressing his skepticism. Born had proposed a radical new way of thinking about matter-waves in quantum mechanics, which had become a de facto standard for the physicists of the time.
‘God Does Not Play Dice with the Universe’
In his letter to Born, Einstein made one of his most famous and often quoted remarks. Let’s take a look at the key sentiment Einstein hoped to communicate through this letter: “Quantum mechanics is very impressive. But an inner voice tells me that it is not yet the real thing. The theory says a lot, but does not really bring us any closer to the secret of the old one. I, at any rate, am convinced that He does not play dice.” Einstein expressed sentiments similar to this on many occasions throughout his life. In various ways, Einstein became fond of insisting that “God does not play dice with the universe.”
What exactly did Einstein mean when he said this? To understand the nature of Einstein’s objection, it is important to appreciate the role that determinism plays in classical physics. According to the equations of classical physics, in principle, all future events can be calculated and predicted with perfect accuracy.
If it was possible to know the exact location and velocity of every atom and every other particle in the universe, and if one had access to an infinitely powerful computer, then the equations of classical physics could be used to work out everything that would ever happen in the future. It would also be possible to run these equations backward to work out everything that had ever happened in the past. In effect, in classical physics, the future is strictly determined by the present. The universe is like a great and complex clock, simply ticking forward in a complicated, but entirely predictable manner. Einstein subscribed to this vision of the universe.
According to classical physics, there’s no role for chance or probability. The God of classical physics does not play dice. Born’s interpretation of quantum mechanics presented a different image. In it, the universe was not as predictable as it was in classical physics.
Learn more about Einstein’s Rejection of Black Holes.
Quantum Objects Are Simultaneously in All Locations
According to Born and his way of thinking about quantum mechanics, an electron (or any other quantum object) is extended across the volume of space that is covered by the wave function. When we measure the location of an electron, it is always point-like, without any spatial extent.
However, before being measured, an electron is simultaneously in all of the locations covered by its wave function. It effectively is in many places, and all at the same time. In this sense, electrons and other quantum objects have a kind of probabilistic existence, being in all possible places and doing all possible things, at all possible times.
This can also be described in terms of the Heisenberg uncertainty principle. According to this principle, the more precisely defined the location of a quantum object is, the less specified is its velocity, and vice-versa. So, what quantum mechanics says is that an object cannot be in only one place and moving at only one speed.
Imagine an electron. It’s described by a wave function that peaks sharply at two places, which we will name locations A and B. Now, let’s assume that the shape of the wave function covers locations A and B equally, and to the same extent. In this case, if an experiment is carried out to measure the location of the electron, there would be a 50% chance that it would be found at location A and a 50% chance that it would be found at location B.
This example may seem identical to a coin flip, which presents two equally likely outcomes. However, such a comparison would fail to take into account something important about the role probability plays in quantum mechanics.
When a coin is flipped and covered, it’s possible to liken it to the electron mentioned above. It could be heads or tails. The crucial difference is that at the moment the coin is covered it’s already configured itself to either heads or tails, we just don’t know it yet. Whereas, the electron in the above mentioned experiment, is simultaneously present in both locations A and B.
This experiment illustrates that even if we know everything about the electron, even if we know the exact shape of the electron’s wave function, there is still a 50% chance that it would be found at location A, and a 50% chance that it would be found at location B. So, the universe isn’t behaving deterministically.
Learn more about Our Random World and Probability.
Real Indeterminism Is What Bothered Einstein
The quantum universe is fundamentally probabilistic, unlike the deterministic universe described by classical physics. Einstein believed that the universe and its laws must be strictly deterministic. He felt that there could be no role for probability or chance, in nature’s foundation. This is why Einstein didn’t accept or agree with the theory of quantum mechanics.
It, however, needs to be pointed out that Einstein was perfectly comfortable with the roles that chance and probability play in physics. Practical indeterminism such as that presented by the flip of a coin didn’t bother him at all. He was well aware that there was no practical way to predict how the coin would land. He was also aware that even an extremely tiny change in how the thumb struck the coin, or a microscopic alteration to the shape of the coin, or a slight change in the distribution of the surrounding air molecules could change the outcome of any given coin flip.
What did bother Einstein was the prospect of real indeterminism, built deep into the underlying laws of physics. This kind of indeterminism would make it impossible for even an all-knowing being to perfectly predict the outcome of any event. It was this kind of indeterminism that seemed to be present in the new theory of quantum mechanics.
Einstein held on to the belief that the universe was deterministic in nature, and that determinism was built into the fabric of nature itself. This stopped him from agreeing with the consensus building around the probabilistic nature of the universe. Over the next few years, Einstein made several attempts at finding flaws in this theory of quantum mechanics.
Common Questions About Quantum Universe, Determinism, and Probability
The philosophical belief of determinism was first developed by pre-Socratic Greek philosophers such as Heraclitus and Leucippus, between the 7th and 8th century B.C. In later years, Aristotle built on it, but it was the Stoics who made the most significant contributions and also popularized this philosophy.
The philosophy of determinism claims that all events in the universe are determined by preexisting causes, including moral choices. This means that there’s a cause-and-effect relation between all events in the universe. It also means that human free will doesn’t actually exist.
One of the key implications of the Heisenberg uncertainty principle is that it highlights the probabilistic nature of the universe. According to this principle, the more precisely defined the location of a quantum object is, the less specified is its velocity, and vice-versa. So, quantum objects, in effect, have a kind of probabilistic existence.
Wave function (Ѱ) describes the probability of a particle being present at a particular location at a given time. It is also referred to as Probability Amplitude. Quantum objects are present in all possible locations described by the wave function, at all possible times.